This invention relates to shearproof punches which have heel elements forming part of the punch but are biased ahead of the punch to engage with the companion female die when only a portion of the punch is actually cutting for preventing transverse shearing movement of the punch with respect to the die.
Such punches are also called nibbling or notching punches and are used on relatively small presses for notching the edge or punching a hole in a metal sheet, the hole being larger than the cutting portion of the punch, by a plurality of strokes of the punch, the work being moved with respect to the punch and die between strokes or the punch and die being moved with respect to the work between strokes.
The bottom or cutting face of prior art punches have been rectangular, square, round or with rounded sides. Each punch side is provided with a vertically extending groove, usually at the center of the side in which a heel is adapted to slide. Each heel, at least along a portion of its length, is cylindrical with its outer side flatted, or rounded on a greater radius than the radius of the groove, the groove conforming to the cross-sectional slope of the cylindrical portion of the heel and the flatted or rounded face of the heel conforming to the shape of the punch side.
A spring for each heel is provided for biasing the heel downward and an upper stop is provided above each heel for limiting the upward movement of the heel and a lower stop is provided for limiting the downward movement of each heel with respect to the body of the punch. The heel may be headed to provide a shoulder for engaging this lower stop.
When a heel is raised against the upper stop because of the lower end of the heel meeting a portion of the work during a working stroke of the punch the lower working surface of the heel becomes a part of the working surface of the punch and the heel must be aligned so that its flatted or rounded side is aligned with the punch body side. Similarly, when the heel is not raised by contact with a portion of the work, its lower end enters the female die and means must be provided to keep the heel aligned and prevent it from turning so as to injure the die.
In U.S. Pat. No. 3,348,443 to Smith et al, Oct. 24, 1967, the means for preventing rotation of the heel is a circular strap guiding the arcuate outer face of the heel or four flat straps guiding the flatted side of the heel, the straps being bolted to the punch body. These straps limit the useful length of the punch, interfering with the operation of stripper mechanism, if used, and cutting down the useful life of the punch since the lower faces of the punch body and heels must be ground flat to sharpen the punch. Moreover, the strap securing bolts tend to work loose during operation of the punch.
In U.S. Pat. No. 3,742,804 to Patterson, Sept. 18, 1973, pendant spring fingers bearing inwardly extending pads with a flat surface bearing against the flatted sides of the heels are provided to keep the heels from turning. Similarly, in U.S. Pat. No. 2,997,907 to Constantino, Aug. 29, 1961, the heels themselves are provided with inwardly extending pads having a flat surface bearing against a flat surface provided on the punch body. In this type of means for preventing the turning of the heels, hardened steel punch surfaces working against unhardened surfaces cause excessive wear and rapid deterioration of the punch parts. In the latter device, bolted-on parts of the punch body tend to shake loose.
In U.S. Pat. No. 3,269,240 to Killaly et al, Aug. 30, 1966, axially extending narrow grooves are provided in the heels and spring biased aligning pins, carried in the punch body, extend into the grooves to prevent the heels from turning. This type of aligning means requires a proliferation of parts and it has been found that the narrow grooves and necessarily small aligning pins become clogged with debris under working conditions.